1. Field of the Invention
This invention relates generally to cavitating venturis and, more particularly, to small cavitating venturis designed to operate at low Reynolds number (Re) flows of less than about 60,000.
2. Discussion of the Related Art
Cavitating venturis are widely used for the purpose of controlling liquid flow rates in fluid flow systems. Essentially, a venturi is a nozzle having a minimum area throat section between two tapered sections. Specifically, the typical textbook venturi is comprised of a long conical converging section in which the fluid total head is converted to a velocity head, a minimum area throat in which the fluid static pressure is equal to or less than the fluid vapor pressure, and a shallow angle conical divergent section in which the fluid velocity head is converted back to pressure head in a low-loss process. In other words, the throat diameter of the typical cavitating venturi is sized such that the static pressure of the fluid is equal to or below the vapor pressure of the flowing fluid, thus causing the fluid or liquid at the throat to form gaseous phase bubbles which travel at sonic speeds.
By allowing the flowing liquid to vaporize or cavitate at the nozzle throat, the influence of downstream pressure variations on flow rate is eliminated. That is, fluid flow rate is no longer dependent upon the pressure difference across the venturi, but is dependent upon upstream pressure alone. Once this condition occurs, the flow rate and upstream pressure are independent of the downstream pressure. In the typical textbook, high flow, high Reynolds number (i.e. Re greater than 60,000) cavitating venturi design, this condition of cavitation and flow control can be maintained with the downstream pressure being as high as 80% of the upstream pressure. In such a case, 20% of the total pressure at the venturi inlet is lost in nonrecoverable losses. The venturi is thus said to have a pressure recovery capability of 80%.
However, when such conventional textbook designs are applied to very small, low flow venturis having a Reynolds number of 60,000 or less and venturi throat diameters of about 0.020 inch or less, serious problems are encountered. Specifically, such venturies have been shown to demonstrate both poor pressure recovery and unpredictable flow control (bistability). Measurements of pressure recovery in which loss of flow control at downstream pressures as low as 50% of the upstream pressure have been observed (i.e. 50% of the total inlet pressure is lost in the process). Bistable operation in which the venturis operate in two distinct modes, differing in flow rate for a given or fixed upstream pressure by as much as 15% is also a common occurance. It is postulated that this bistability results from a hydraulic instability in which the vena contracta (minimum effective area) moves from within the throat area to downstream of the throat in a chaotic unpredictable fashion.
What is needed then is a low flow, low Reynolds number (i.e.: Re.ltoreq.60,000) cavitating venturi which does not suffer from the above-identified disadvantages. Such a design must eliminate the poor pressure recovery, increase flow control at downstream pressures at least as high as 80% of the upstream pressure and prevent the cavitating venturi from becoming bistable or operating in two distinct modes differing in flow rates. It is, therefore, an object of the present invention to provide such a cavitating venturi.